During oil and gas well drilling, at certain depths and at total depth, the operator typically runs and cements a string of casing in the well. The string of casing is made up of sections of pipe, each about 40 feet in length. Each section of pipe has externally threaded ends, one of which has a collar secured to it, the collar having internal threads.
While picking up another section of casing, the operator supports the made-up string of casing in the well with slips at the rig floor. The collar normally has insufficient strength to bear the weight of a lengthy string of casing, thus the slips have to be configured to grip the exterior of the casing below the collar.
Generally the slips comprise a plurality of segments that slide up and down a conical bowl between released and gripping positions. Power slips employing hydraulic cylinders to move the slip segments are commonly employed. Power slips can be mounted to a rotary table at the rig floor. In that instance, as the operator screws the new section of casing to the string of casing supported by the slips, the torque imposed on the string by the make-up mechanism can be transferred through the slips to the rotary table to prevent the string from spinning during make-up. Power slips can also be employed to support a string of drill pipe.
Power slips are also used as elevators. In that instance, they are coupled to elevator bails, which are supported by a lifting mechanism on the drilling rig, such as the blocks or a top drive. When used as elevators, they move up and down the derrick with the lifting mechanism and are used to lift sections of casing to be made up with the made-up string.